It is known to use acid-functional acrylic resins (e.g., S.C. Johnson SCX.TM.-815, 817 and 819) cured with triglycidylisocyanurate (TGIC) or with hydroxyalkylamides as coating powder compositions. These coatings have many desirable properties such as: high hardness, good clarity, good chemical resistance, and excellent resistance to loss of gloss on outdoor exposure. A typical shortcoming of these coatings is low flexibility, and poor impact resistance.
"Hybrid" coatings have also been prepared in which the acrylic resin is blended with epoxy-functional resins, especially those derived from epichlorohydrin and bisphenol A. These coatings have many desirable properties, including good flexibility and impact resistance. However, they lose gloss in less than 500 hours when exposed to simulated outdoor weathering.
Coatings prepared from epoxy-functional acrylics, such as those from acrylic copolymers containing glycidyl methacrylate, are known to exhibit similar properties to those prepared from the acid-functional acrylics mentioned above. When cured, for example, with dodecanedioic acid, they are hard, clear, smooth, chemically resistant, and resistant to weather. They also exhibit the same drawback as the coatings from acid-functional acrylics, i.e. poor flexibility and poor impact resistance.
U.S. Pat. No. 3,817,946 discloses that the flexibility of glycidyl methacrylate-modified acrylics can be improved if a part of the polycarboxylic acid curing agent is replaced with a semicrystalline acid-functional polyester. Such materials were found to have two shortcomings. First, the powder showed a tendency to sinter. Second, the improvement in impact resistance and flexibility was marginal.
It is a general object of the present invention to provide a coating powder composition having resistance to weathering, and at the same time, good flexibility and impact resistance.